Molecular frequency standard

ABSTRACT

A molecular beam tube frequency standard using a barium oxide molecule of the form Ba138016 having spinless atoms and using electrostatic state selection. The beam tube comprises a beam source including a thin-walled refractory electrically conductive oven tube containing Ba138016. The oven tube, which is chemically inert to barium oxide, is surrounded by a thermionic electron emitter which is made negative with respect to the oven tube. Electrons emitted from the electron emitter bombard the oven tube and heat the barium oxide to a temperature at which Ba138016 molecules are evaporated. The beam tube also comprises a cavity resonator supporting a TM mode which includes a cylinder and two end pieces provided with apertures for passage of the beam. Each of the ends of the cylinder is provided with a barbed edge; a flat surface of each end piece is pressed against a corresponding barbed edge of the cylinder by mounting means so that the barbed edges cut slightly into the surface of the end pieces. The beam tube also includes a detector comprising a heated elongated member stretched along the axis of the beam tube and arranged to intercept beam particles over a focal region extending along a portion of the length of the said member. Ions are formed in the vicinity of the elongated member and are collected by electrode means maintained negative with respect to said member.

United States Patent [72] Inventor l-lelmut W. Hellwig Oakhurst, NJ. [21] Appl. No. 721,776 [22] Filed Apr. 16, 1968 [45] Patented May 18, 1971 [73] Assignee The United States of America as represented by the Secretary of the Army 54] MOLECULAR FREQUENCY STANDARD OTHER REFERENCES Microwave Spectrum of BaO" By L. Wharton et al. From The Journal of Chemical Physics," Vol. 38, No. 11, June 1, 1963, Pages 2705- 2708.

Primary Examiner-William F. Lindquist Attorneys-Harry M. Saragovitz, Herbert Berl. Edward J.

Kelly and Daniel Sharp ABSTRACT: A molecular beam tube frequency standard using a barium oxide molecule of the form Ba O having spinless atoms andusing electrostatic state selection. The beam tube comprises a beam source including a thin-walled refractory electrically conductive oven tube containing Ba 0 The oven tube, which is chemically inert to barium oxide, is surrounded by a thermionic electron emitter which is made negative with respect to the oven tube. Electrons emitted from the electron emitter bombard the oven tube and heat the barium oxide to a temperature at which Ba 0 molecules are evaporated. The beam tube also comprises a cavity resonator supporting a TM mode which includes a cylinder and two end pieces provided with apertures for .passage of the beam. Each of the ends of the cylinder is provided with a barbed edge; a flat surface of each end piece is pressed against a corresponding barbed edge of the cylinder by mounting means so that the barbed edges cut slightly into the surface of the end pieces. The beam tube also includes a detector comprising a heated elongated member stretched along the axis of the beam tube and arranged to intercept beam particles over a focal region extending along a portion of the length of the said member. Ions are formed in the vicinity of the elongated member and are collected by electrode means maintained negative with respect to said member.

POTENTIAL ENERGY OF MOLECULE Patented May 18,1971 3,578,968

3 Sheets-Sheet 1 E"* ELECTRIC FIELD H MAGNETIC FIELD iglfllllllg ooqanlo. 0'00... s

OVEN l2 FIRST RESONATOR SECOND HOT WIRE HEATER COIL I4 ENERGY STATE CAVITY ENERGY STATE DETECTOR AND SELECTOR SELECTOR MULTIPLIER ELECTRODES EL ECTRODES INVENTOR, HELMUT W. HELLWIG.

A T TORNE YS Patented Ma 1s,1971 I I 3,578,968

5 Sheets-Sheet 2 ass INVENTOR. HELMUT W HELLWIG.

A TTORNEYS Patent ed May 18,1971 3,578,968

5 Sheets-Sheet 3 MICROWAVE FROM GENERATOR OUT PUT INVENTOR, HELMUT W. HELLWIG.

BY M w AT TORNEYS MOLECULAR FREQUENCY STANDARD The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.

BACKGROUND OF THE INVENTION One of the principal disadvantages of atomic beam controlled frequency standards, such as the cesium atomic beam resonance device, is that the transition frequency is affected by external magnetic fields, for example, the earth's magnetic field, to which the device is always subjected. Such atomic devices, often erroneously referred to as molecular beam devices, cannot adequately be shielded from these external magnetic fields and therefore are subject to the Zeeman effect associated with said magnetic fields, unless the cost and size of the overall device are unduly increased toprovide adequate shielding.

The use of a molecule in such frequency standards yields certain advantages. The dipole moment of a molecule is approximately 1,000 times greater than the dipole moment of the atom. Since the optimum power in a beam tube depends upon the square of the dipole moment, a molecular beam tube will require about 1,000,000 times less electromagnetic driving energy than an atomic beam tube of comparable design.

The influence of external magnetic fields on the transition frequency in the case of the usual type of molecule which has no electronic angular momentum is much less than in the case of the atom. The Zeeman effect is decreased at about the ratio (about 1,836) of the Bohr magneton to the nuclear magneton, thus making the molecular transition significantly less sensitive to magnetic fields than the atomic hyperfine transitions presently used in frequency control. Although this feature is traded off against a much increased sensitivity to electric fields, this creates no special problems since electric fields are much more readily shielded against than magnetic fields. Electrostatic state selectors are resorted to in the case of molecular beam tubes in contrast with the much bulkier and heavier magnetic state selectors used in atomic beam tubes, thus reducing the weight and size of a molecular beam frequency standard.

Having established the advantages to be gained by using a molecular beam frequency standard rather than an atomic beam frequency standard, selection of a particular molecule is v the next consideration. The use of a molecule composed of spinless atoms provides an easy way to avoid severe impairment of frequency accuracy owing to a quadrupole hyperfine structure or magnetic hyperfine structure. In the barium oxide isotope Ba O' no quadrupole or magnetic hyperfine energy state is present.

The rotational states of the diatomic Ba O molecule will split symmetrically in the presence of an external magnetic field. The lower of the two levels will have no splitting, whereas the upper level will split symmetrically into three sublevels. The potential energy for one of the sublevels does not change with external field strength, while the slope of the energy vs. field relationship for the other two are equal and of opposite sign and are symmetrically arranged with respect'to the first sublevel. The two sublevels whose energy changes with magnetic field strength are equally populated so that there will be virtually no frequency shift owing to external magnetic fields. Although there are no magnetic moments associated with the components of the Ba O molecule and no decoupling effects can occur which usually lead to a quadratic field dependence, the diamagnetic properties of barium oxide give rise, at high magnetic field strengths, to a second order Zeeman effect or quadratic field dependence arising from the which would likely be encountered, even without any magnetic shielding of the beam tube.

In order to minimize the effect of beam velocity on the accuracy of the frequency standard, the beam velocity should be as small as possible. The relatively heavy weight of the Ba O molecules is a factor in achieving a relatively low molecular beam velocity of the order of 4X10 centimeters per second.

In the case of the Ba O' molecule, the transition frequency of 18.7 GHz. is within the promising frequency range of about 10 to 30 GHz. because this frequency range yields a relatively good line-Q while still permitting a cavity type resonator of reasonable size and use of well-known microwave generators of high efficiency and performance rating, By way of comparison a CO beam has a much higher transition frequency of l 15 GHz. and would impose greater restrictions on electromagnetic generators and resonators.

The high evaporation temperature of barium oxide, which is a solid at ordinary room temperatures, results in a vapor pressure at ambient temperatures which is extremely low. Consequently, vacuum problems with this particular barium oxide molecular beam tube are virtually nonexistent; in contrast, many molecules exist in the gaseous phase at ordinary room temperatures.

Another advantage of the Ba O molecule is that it is readily, detectable with the highly efficient and simple hot wire ionizer type of detector.

In order to obtain adequate beam strength, a relatively high vapor pressure of about 10 Torr or greater is needed; this, in turn, requires a relatively high temperature (of the order of 1,500 C.) in the case of barium oxide. For a practical molecular beam frequency standard, the molecular beam generating oven must be of small size and low energy consumption. The oven itself is an iridium tube containing the barium oxide in solid form; the iridium tube is closed off except for a small opening in one end to allow exit of the molecular beam. The iridium tube is surrounded by a conventional thermionic emissive low voltage heater element which is made negative with respect to the iridium tube by means of an appropriate low voltage power supply expending about 50 watts. Electrons emitted from the heater element bombard the positive oven tube, thereby heating the thin oven tube and the barium oxide contained therein to a temperature sufficient to evaporate off molecules of barium oxide. Radiation heat losses are minimized by a series of surrounding heat shields and conduction heat losses are limited by use of a thin-walled refractory tube for mounting the iridium oven tube. Iridium is chosen for the oven tube because it is a highly refractory metal which is chemically inert to barium oxide. The heater tube must be a metal since it must be electrically conductive to attract electrons emitted from the thermionic heater element; this rules out such refractory materials as ceramics. Since tube temperatures of the order of 1,500 C. or greater must be obtained to evaporate the barium oxide, the oven tube must be made of a metal having a melting point in excess of l,500 C. Use of a refractory metal such as tungsten is precluded since such a container would disintegrate from the oxidation reaction occuring between tungsten and barium oxide. An iridium oven tube, on the other hand, is not contaminated or physically destroyed by contact with the evaporating Ba O The molecules of proper energy level pass through a microwave cavity resonator where they interact with an electric field within the resonator. The use of a single (Rabi) cavity resonator often is more desirable than the double (Ramsey) cavity resonator often used in beam tubes since it will result in size and weight reductions in the beam tube, as well as a much simplified electrical circuit for frequency control. For simplicity of construction, the cavity resonator comprises a hollow cylinder to the ends of which are attached end pieces centrally apertured to allow passage of an axially directed molecular beam. In order to achieve maximum interaction of a symmetrical axial molecular beam (this is a beam which is readily formed) with the electric field in the cavity resonator,

the electric field lines should coincide with the axis of the resonator. With this particular TM mode of operation relatively large currents flow along the wall of the cavity resonator and over each junction between the resonator cylinder and the corresponding end piece. The principal obstacle to the use of the Rabi resonator is the practical difficulty in approaching the ideal design, since any mechanical distortion will result in phase and/or Doppler shifts of the atomic or molecular frequency. For example, any mechanical distortion such as poor contacts or variable contacts, or fillets along the circumference formed during brazing, will change the conductivity and affect the wall currents. thereby causing the aforesaid undesirable frequency shifts. To minimize these distortions which would occur normally at the cylinder-end piece junctions, the ends of the cylinder are designed with sharp circumferential edges. The flat surface of the corresponding end piece is pressed evenly against the sharp circumferential edge of the cylinder by mounting means such as screws, allowing the edge to cut slightly into the corresponding end piece. In this manner, no uncontrollable welding deformations and no additional material such as brazing fillets with resulting variations in conductivity, are introduced. This design, consequently, will reduce distortions such that the Rabi cavity resonator becomes feasible for use in molecular beam frequency standards.

One conventional method of detecting atomic beams is by stretching the detector or ribbon perpendicular to the direction of propagation of the beam. The multipole beam optics used in the present molecular beam tube display axial symmetry and the beam cross section is circumferential. Consequently, a transverse wire does not intercept a substantial portion of the beam and the detector is relatively insensitive. The detector used in the beam tube of the subject invention includes a hot wire ionizer stretched along the beam axis. This ionizer may include a single wire, but preferably comprises two wires twisted together and separated at one end, thus permitting entrance of the molecular beam and facilitating mounting of the wire ionizer. The axial molecular beam eventually becomes convergent and the focal point is located on the wire. Practical beam tubes, however, are characterized by beam velocity variations which cause the various beam particles to be deflected from the axis of the tube by different amounts. Molecules of lower velocity, for instance, will have lower kinetic energy and will be deflected from the axis by a greater amount than molecules of higher velocity. The amount of deflection also depends upon the voltage of the state selector. This beam velocity variation thus gives rise to aberration or a defocusing effect and thus requires a multifocal detector. In other words, a focal region exists along the axis of the beam the length of which is proportional to the velocity spread of the various beam particles. With the axial arranged wire ionizer of the invention, therefore, substantially all beam particles of the selected state will be focused on the ionizer, with greatly increased efficiency and sensitivity.

When a beam of barium oxide molecules is used the impingement of such molecule on the hot wire ionizer causes a reduction to barium and oxygen atoms. The oxygen acts as a catalyst to provide a reaction wherein the barium atoms are converted into barium ions and electrons. This catalysis greatly assists in ion generation and increases the detector sensitivity. The barium ions are collected by a portion of the detector structures, such as a cylinder surrounding the wire, which is negative with respect to the wire. Substantially all of the oxygen generated during impingement of the molecules on the surface of the hot wire is retained in the hot tungsten wire which oxidizes during bombardment by the barium oxide molecules. Consequently, no need exists for outgassing of the tube at the detector end.

SUMMARY OF THE INVENTION A molecular beam tube frequency standard using a barium oxide (Ba O molecule having spinless atoms and using electrostatic state selection. In the presence of external magnetic fields, the Zeeman effect causes symmetrical splitting of the upper energy transition level of such molecules into three sublevels, the middle one being independent of any external magnetic field and the other two being equally dependent upon the magnetic field with opposite sign. Since electrostatic (nonmagnetic) state selection can be used, the two magnetic field dependent sublevels will always be equally populated; this effect, coupled with the symmetrical Zeeman splitting, makes it possible to avoid frequency shift owing to external magnetic fields.

The molecular beam tube includes a beam source comprising a thin-walled refractory oven tube of iridium containing barium oxide to be evaporated. The iridium oven tube, which does not react chemically with the barium oxide, is surrounded by an electron source such as a tungsten cathode which is electrically heated. A direct current voltage is applied between the source and the oven tube such that the oven tube forms the anode; consequently, electrons from the cathode bombard the oven tube and the material in the oven tube is heated by electron bombardment, whereby a beam of molecular barium oxide is produced.

The molecular beam, after appropriate state selection, is directed through a cavity resonator which is arranged to support a TM mode. The resonator includes a cylinder with two end pieces provided with holes for passage of the beam. In order to minimize frequency-shifting distortions caused by irregularities in the cylinder-end piece junctions, each of the ends of the cylinder is provided with a sharp edge and the flat surface of each end piece is pressed against one of the sharp edges by mounting screws, assuring even contact since the sharp edge cuts slightly into the material of the respective end piece.

After passage through the cavity resonator and a second state selector, the beam is detected by a hot wire ionizer of axial symmetry. The wire ionizer is stretched along the axis of the molecular beam. The ionizer can comprise, for example, a pair of twisted wires separated at one end to permit entrance of the beam. The focal point of the converging beam can be located anywhere along the whole length of the detector wire, thus providing simultaneous detection of all velocities present in a multivelocity beam.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1A and 1B are graphs showing the Stark effect and Zeeman effect, respectively, of the lowest rotational transition from 1 1 to J=0 for the Ba O molecule at about 18.7 GHz.;

FIG. 2 is a view showing schematically a molecular beam tube according to the invention;

FIG. 3 is an exploded view showing the relationship of the principal elements of the molecular beam tube of FIG. 2;

FIG. 4 is a view showing a beam generating source according to the invention;

FIG. 5 is a view showing in detail the heater wire assembly of the beam generating source of FIG. 4;

FIG. 6 is a view illustrating a typical state selector for use in the molecular beam tube of FIGS. 2 and 3;

FIG. 7 is a view showing a beam tube cavity resonator in accordance with the invention; and

FIG. 8 is a view of a hot wire ionizer detector according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1A and 1B illustrate, respectively, the Stark and Zeeman effects for the lowest rotational transition from the J=l to .I=O energy levels or states for the Ba O molecule. As shown in FIG. 1A, in an electric field the upper rotational level splits into two sublevels M=0 and lMl =1. The relative frequency shift Af/f owing to the Stark effect, for the J=l, M=0 state, the 1:1, lM l=l ;ate and the 1:0, M=0 state can be calculated for the Ba b molecule. The presence of an electric field of strength E will change the energy of each of the molecular sublevels by an amount AW. Being a linear molecule, Ba O has no electrode dipole moment which is fixed in direction and no first order Stark efiect is present. The second order Stark effect produced by an electric field E can be given by the relation where p. is the dipole moment, h is Plancks constant, B is the rotational constant, J is the rotational quantum number, M is the projection of J on the direction of electric field, (M=0, +1 or 1 Since this is a second order effect, only the absolute value of one is significant since the square of or 1 is one. From the equation for C the values of C C and C are 0.10, 0.05 and O.l7, respectively. For Ba O which has an electronic dipole moment of about 8X10 c.g.s. units and a rotational constant of O.9XHz., the constant k is approximately l0 The frequency shift Af/ f is given by the relation and m where f is the resonance frequency for Ba O equal to approximately 18.7 GI-lz. From this equation, and from the values of C C and C the values of Af/f are approximately 8 l0 E and 4Xl0 E for. the upper and lower J=l sublevels, respectively and l4 l0 E for the lower (J=0) level. The maximum frequency shift of the 1,0 transition owing to an electric field thus is seen to be about 24.4 1OI5 where E is in volts per centimeter. For a frequency stability of 10", for example, where one would need to avoid an electric field E greater than 4 l XIOequals approximately 7 millivolts per centimeter. The oscillating microwave field introduced into the cavity resonator will cause a complicated Stark effect but the maximum Stark effect frequency shift owing to the microwave field, considering E as the electric field amplitude at optimum power, (which for one working model of the Ba O beam tube was about 4.2Xl0 watts), would be only about 4X10? The Ba O molecules in the upper sublevel J=l, M=0 can be focused into the microwave cavity resonator whereas the molecules in the lower sublevel J=l, M=l and in the lower rotational state J=0 will be deflected away from the cavity resonator.

Because of the relatively high Stark coefficients, state selection is rendered easy. However in order to satisfy stability requirements, it is preferable to operate at relatively low electric fields, for example, at fields below 10 volts per centimeter where the two Stark sublevels of the upper rotational state coincide within a corresponding relative frequency deviation ofless than 10.

The atoms Ba and O of which the barium oxide molecule is composed have no spin and therefore no magnetic moment of their own. The only magnetic moment associated with this particular barium oxide molecule is one resulting from its rotation. The rotating molecule therefore will adjust its orientation in an external magnetic field such that the projection M (a different parameter than the M mentioned in connection with the Stark effect) of the rotational moment (quantum number J) on the direction of the magnetic field is an integer. This projection M can have the values 0 and plus or minus 1. Each value of M corresponds to energy level with a magnetic field dependent energy offset AW from the value at a magnetic field strength H=O which is given by the following equation AW=p.,,gMH, where [L is the nuclear magneton, and g is the magnetic g-factor of the molecule which for Ba O is equal to 0. 103.

FIG. 1B shows the Zeeman effect of the two rotational levels for the Ba O molecule. Although other levels exist, they are removed so widely from the microwave region as to be of no practical significance in the molecular beam tube of the subject invention. In the cases which are of interest here, the J=0 level will have no splitting, whereas the J=l level splits into three levels, as indicated in FIG. 1B. The frequency shift 1 Af for the M= ':l sublevels is equal to AW/h. It is given by The slope of the M=il sublevels thus is 79 hertz per Gauss. The 1,0 transition will split into three equally spaced lines with their center frequency being unshifted at the frequencyof thezero field transition. Although the line intensity of the M=0, M= '-l sublevels differ somewhat, the population of the M= *-1 and M=-l sublevels will be equal, for all reasonable magnetic field strengths. This is important, since a frequency shift has been found to occur owing to a population difference, as well as from asymmetrical splitting. This equality in population results from normal thermal equilibrium conditions which predict that all levels will be equally populated and from the fact that this thermal equilibrium will not be upset at the electrostatic state selector since no magnetic forces will be acting upon the molecular beam particles. This is in contrast with the usual atomic beam tubes where magnetic state selectors are used for state separation. Because of the energy vs. magnetic field symmetry of the M=+l and M=-l sublevels; in other words, since the slopes of the two sublevels in FIG. 1B are equal and opposite, there will be no resultant J=1 energy level change AW with changing magnetic field strength H, and further, since there is no substantial J=0 energy level change with changing magnetic field strength I-I, there will be virtually no frequency shift in the J=l to J=O transition owing to external magnetic fields.

Although there are no magnetic moments associated with the components of the barium oxide molecule and no decoupling effects which usually lead to a quadratic field dependence, the diarnagnetic properties of the molecule must be considered to provide a conclusive treatment of the Zeeman effect. The rotating molecule tends to compensate the external magnetic field by a so-called Larmor precession which causes a magnetic moment and an energy change which is equal to the product of this magnetic moment and the strength of the magnetic field.

It can be shown that the frequency Av obtained because of the Larrnor precession is Al 2h the magnetic field H has to be This field roughly is 1,000 times the average strength of the earths magnetic field, so that the latter would not substantially effect the frequency of the molecular beam tube. In contrast, the cesium atomic beam tube would require that any external magnetic field be less than about 0.004 Gauss times less than the earths magnetic field) in order to have an absolute frequency offset of less than 10 This obviously would mean that expensive and bulky shielding means would have to be used .with a cesium atomic beam tube in order to 2 X 10 450 Gauss achieve the same frequency stability that could be achieved with the molecular beam tube of the subject invention without any external shield whatsoever. Furthermore, a magnetic material such as invar, which has a very low thermal coefficient of expansion, can be used with a barium oxide beam tube; in contrast, the cesium atomic beam tube cannot use such a material for the cavity resonator since the cesium beam cannot tolerate the presence of relatively strong, inhomogeneous magnetic fields.

FIG. 2 illustrates schematically a molecular beam tube 5 according to the invention. An exploded view of the tube, with only a portion of the envelope, is. shown in FIG. 3. The molecular beam is generated by evaporation of Ha -O in an oven which comprises an iridium heater tube 12 containing the barium oxide in solid form. The heater tube 12 is surrounded by heater wires 14 which is negative with respect to the heater tube so that the electrons emitted from the heater wire bombard the heater tube 12 to heat the latter to the required evaporation temperature of the barium oxide. The molecular beam then enters a first state selector 15 which can be of the quadrupole type designed for optimum focusing of the .l=l, M=0 molecules with a most probable speed of about 4.3Xl0 centimeters per second (corresponding to an oven temperature of l,500 C.) when operated at 5 kilovolts. As shown in FIG. 6, this quadrupole focuser or state selector 15 comprises four equally spaced wires or rods 66 with alternating ones being interconnected. A high voltage is applied in push-pull to the alternate electrodes or wires which are brought out to terminals 72 angularly displaced about one of the two annular ceramic spacers 68 and 69. The molecule of the selected state are concentrated along the axis of the tube by the focusing effect produced in the state selector 15. The other states of lower energy level are deflected away from the longitudinal axis of the beam tube 5 and do not enter the cavity resonator. lnput microwave energy of the proper transition frequency (18.702 GHz.) is supplied to a Rabi cavity resonator by way of a waveguide 93 from any conventional microwave generator such as a klystron, not shown. This resonator 20 is designed to operate in the TM mode so as to obtain substantial interaction between the electric field within the microwave cavity resonator and the molecular particles passing through the cavity resonator. The latter is designed, in a manner already described in some detail, to minimize mechanical and electrical irregularities which tend to produce phase and Doppler shifts within the cavity resonator. The cavity resonator 20 is supported within the envelope 9 of the tube 5 by ceramic members 85 and 86. The Ba O molecules, upon leaving the cavity resonator, pass through a second electrostatic state selector or focuser (see H08. 2 and 3) which may be identical to the first state selector 15. The molecules of the desired .l=l, M=0 state are converged by the state selector 25 onto the axis of the molecular beam tube while molecules of other energy levels are deflected away from the axis of the molecular beam tube 5. Because the molecular beam has a Maxwell velocity distribution, the second state selector 25 will have a focal region along the axis of the beam tube rather than a single focus. A substantial portion of the molecules of the selected state impinge upon a hot wire ionizer detector which comprises an axial wire 102 (or wires 102, 103, in the version shown in FIG. 6) heated to a relatively high temperature to increase the speed of response. A substantial portion of the barium oxide molecules impinge upon a focal region along the axially arranged wire of the detector 30 and the ions resulting from said impingement are collected for example, by cylinder 107 (see FIG. 6) to which an output lead 34 is connected. The entire detector assembly 30 includes two supporting discs 35 and 36 for mounting the detector within the envelope 9 of the beam tube 10. An output current is provided which is a minimum whenever the cavity resonator excitation coincides with the resonant transition frequency.

FIG. 4 illustrates a beam generating oven 10 in accordance with the invention. The oven itself is a small, thin-walled iridium tube 12 closed at one end 39 and a centrally apertured plug 41 serving as a chamber for retaining the Ba' O material to be evaporated. The reasons for the choice of iridium for the oven tube already have been set forth. The plug 41 is centrally apertured to allow exit of the molecular beam. The iridium oven tube 12 is fixedly attached to an elongated hollow support tube 44 made of a refractory metal such as tantalum. The support tube 44 is thin-walled so as to minimize conduction heat losses from the iridium tube 12. The support tube is attached to the oven end plate 42, which also serves as an end inclosure for the molecular beam tube 5; this end plate 42 can be made, for example, of stainless steel. The support tube 44 also provides support for the heater wire assembly 45 which surrounds the iridium oven tube 12. The oven tube 12 is recessed in the heater assembly 45 so as to provide for maximum heating at the oven tube aperture. The heater wire assembly 45 includes a hollow cylindrical member 46 of a refractory metal such as tantalum with mica inserts 47 and 48 at each end. The support tube 44 passes through a central aperture in mica insert 47 and is supported therein. The heater wire 14 is in the form of a cagelike structure in which the heater includes a plurality of longitudinally arranged portions which pass through one pair of aligned apertures in the mica spacers 47 and 48 and loop over a mica space to the next pair of aligned apertures, as shown in FIG. 5. One end 50 of heater wire 14 is connected by way of one heater terminal 53 to the negative terminal of an external direct current power supply 55 through one end of transformer 56. The other end 51 of the heater wire 14 is connected by way of a terminal post 54 to the opposite end of the aforesaid transformer. The positive terminal of the power supply 55 may be connected directly to the envelope 9 of beam tube 5 which is in electrical contact with the support tube 44 and oven tube 12. The terminal post 53 and 54 are brought through the end plate 42 by respective electrically insulating bushings 58 and 59.

In order to reduce heat radiation losses from the oven, a tripartite heat shield assembly surrounds the heater wire assembly 45. The composite heat shield includes an inner shield 62 supported from the middle heat shield 63 by electrically insulating supports. The middle heat shield 63 is attached to the outer heat shield 64, as by brazing, and the entire heat shield assembly is supported by connecting an elongated portion of the outer heat shield 64 to the end plate 42 and by means of a thin annular disc 57 which is attached to the inner periphery of the tube envelope 9. The heat shields preferably are made from polished stainless steel and reflect back to the oven a substantial portion of the heat radiated from the oven.

The oven tube 12, together with the support tube 44, are at a positive potential relative to the alternating current heater wire 14. The heater wire 14 can be of the type commonly used in low power indirectly heated vacuum tube rectifiers; the input power required of such a heater wire is only a few watts. A voltage of about 250 volts exists between the heater element 14 and the oven tube 12. Electrons emitted thermionically from the heater wire 14 with a current of about 200 milliamperes are attracted to the positive oven tube 12 and the electron bombardment of this iridium tube is such as to bring the temperature thereof to about l,500 C. This temperature is well below the melting point (2,4l0 C.) of the iridium tube 12. Molecules of lita O evaporated from the oven pass through the exit aperture in the oven chamber at a velocity of about 4.3Xl0 centimeters per second at the temperature of 1,500 C. A satisfactory signal-to-noise ratio is obtainable with 10 molecules per second arriving at the detector. To provide this number of molecules, a typical beam tube would require a molecular flux from the oven source of about l0 molecules per second. The volume of the iridium oven is about 0.25 cu. centimeters so that one charge of Ba O contains about 5 l0 moleculesenough for 17 years of continuous operation at the aforesaid beam intensity. The end disc 69 and the end portions of the three he a; shields 62, 63 and 64 are apertured centrally to permit passage of the molecular beam and also can contain nonaligned apertures to facilitate evacuation of the tube prior to operation. Once a vacuum has been obtained and the tube closed off there is no need for a vacuum Ba O is negligibleof the order of l Torn-and the only gas generating processes in the beam tube would result from outgassing. This can be taken care of readily, however, by insertion of a conventional getter in the vicinity of the oven 10.

The Ba O molecule eminating from the oven assembly 10 next pass through a first energy state selector or focuser shown in FIG. 6 which comprises essentially four electrically conductive wires 66 spaced at angular intervals of 90 about the longitudinal axis of the beam tube 5; only three of the four wires are visible in FIG. 6; the ends of the four wires are shown in FIG. 3. Each wire 66, which may be made of stainless steel, is bent at the ends and inserted into a radially drilled hole 67 in the corresponding one of two annular electrically insulating rings 68 and 69. The portion of the radial hole 67 not occupied by the closely fitting wire is plugged with an electrically insulating material 71 so as to avoid the possibility of arcing between the end of the high voltage wire and the beam tube envelope. Four terminal posts 72, only three of which are visible in FIG. 6, are provided around the periphery of the ring 69 to apply the necessary electrical voltages. Alternate wires are connected together by jumpers 73 and 74. A first lead 75 is brought out from one of the interconnected terminal pairs through the wall 9 of the beam tube 5 through a bushing 76 and second lead 77 is brought out from one of the other interconnected terminal pairs through a bushing 78 also in the wall of beam tube envelope 9. A voltage of the order of 5 kilovolts is applied across the two leads so that alternate wires of the quadrupole state selector 15 are at a 5 kilovolt difference in potential. As previously stated, the quadrupole state selector is an efficient means for focusing the J=l M=0 molecules of a speed, which, for the Ba O molecule at 1,500+ C., is about 4.3 centimeters per second.

The molecules of the undesired states are not focused along the longitudinal axis of the state selector but are'deflected away from said axis. The amount of deflection of the undesired energy states from the beam tube axis is a function of the particular molecule, the velocity of the molecules and the length and radius of the state selector and the state selector voltage. For example, the longer the state selector, the greater the deflection; however, if the length is made too great, troubles arise owing to an increase in the number of molecular collisions.

The Ba O molecule of the proper state pass along the axis of the beam tube 5 through the narrow opening 81 in the cavity resonator 20, shown in FIG. 7. The cavity resonator comprises essentially an elongated cylinder 82, which may be made of any hard metal having a low thermal expansion coefficient, such as invar, to minimize frequency shifts owing to thermal detuning of the resonator and end pieces 83 and 84 of soft metal attached, as by mounting screws 94, to flanged portions of cylinder 82. The cavity resonator assembly 82-84 is supported within the beam tube envelope 9 by electrically insulating spacers 85 and 86 attached to the resonator subassembly, as by set screws 88. The end pieces 83 and 84 have axial holes 81 and 89, respectively, through which the molecular beam enters and leaves the cavity resonator 20. An energy coupling slot 91 is provided in the wall of cylinder 82 and is arranged to receive a waveguide 93 extending out of the beam tube envelope 9. The portion of the waveguide external to the beam tube 5 is connected to a microwave generator such as a magnetron or klystron capable of providing reasonable amounts of energy at the transition frequency of the Ba0 molecules, that is, at approximately l8.7 GHz.

As previously explained, the single cavity or Rabi resonator offer advantages over the Ramsey or double cavity resonator. In designing a single cavity resonator for the molecular beam tube, it is necessary to obtain interaction of the molecular beam with an electrical field. If one were to excite the resonator in a TE mode such as the TE mode, the electric field would be zeroed along the axis and the field maxima would be near both walls. This would necessitate the use of an asymmetrical molecular beam which, in turn, would complicate the beam generating design. In a TE mode, even if an asymmetrical molecular beam were generated, the electric field would vary along the length of the resonator and the abrupt changes of electric field encountered by the moving molecules would result in Majorana transitions and additional Doppler shifts. Consequently, to avoid these spurious level transitions, one

preferably should use the TM mode used with molecular i beams. The preferable TM mode is TM, mode; this mode is such that the electric field lines are along the axis of the cavity resonator, that is, along the direction of propagation of the beam; these lines return along the walls of the resonator.

In designing a cavity resonator, practical fabrication considerations dictate the use of a simple hollow cylinder with the ends of the cylinder being closed by end plates affixed to the ends of said cylinder. The electric currents set up in such a cavity resonator operating in the TM mode are relatively large along the walls of the tube and over the two junctions formed by the cylinder and the corresponding end pieces. In connecting the cylinder with the end pieces, distortions normally have crept in such as poor contact between the edges of the cylinder and the end pieces and conductivity changes such as occur during brazing of the end pieces to the cylinder. These irregularities, particularly when coupled with the high wall currents inherent in the TM mode, give rise to such large phase shifts and Doppler shifts of the molecular transition frequency as to rule out the use of Rabi cavity in molecular beam tubes. In order to overcome these undesirable effects and thereby permit the Rabi cavity to be used in molecular beam tubes of high stability, the two annular edges of the cylinder 82 of the cavity resonator 20 is cut back to form a barbed edge 95 when the flanged end pieces 83 and 84 are tightened against the flanged portions of the cylinder 82. The flat surfaces 96 of the end pieces are evenly pressed against this sharp edge 95 of the cylinder and the end pieces in as much as the sharp edge of the cylinder cuts into the material of the end pieces.

After the interaction of the molecular beam of the desired energy state with the electric field in the cavity resonator, the molecular beam passes through the exit slot 89 into a second electrostatic state selector or focuser 25. This state selector may be identical with the first state selector 15 already described. Again, the action of the second state selector is such as to deflect any molecules of the undesired energy level away from the longitudinal axis of the tube and causes the molecules of the desired energy to converge along said axis.

The molecules of the selected energy state now must be detected and this detection is accomplished by a detector 30 of the hot wire ionizer type. Because of the axial symmetry of all other beam tube elements, the beam focused on the detector also is of axial symmetry. The detector 30, mounted within tube 9 by end discs 35 and 36, comprises a heated a heated tungsten wire stretched along the longitudinal axis of the beam tube. This axial mounting is facilitated if the wire actually includes at least two wires 102 and 103 twisted together. At one end the wires are spread apart so that the aperture in the end disc 105 for entrance of the molecular beam is not obstructed. The detector 30 further includes a cylindrical collector electrode 107 insulated from the end disc by discoidal glass insulating rings 108. The two separated ends of the wires 102 and 103 are mounted by means of respective mounting screws 111 and 112 to the end disc 35. The other ends of the twisted wires 102 and 103 pass through an opening in end disc 36 and are attached by a set screw 113 to a bushing 114 which is spring loaded by spring 115 so as to provide the tension necessary to compensate for thermal expansion of the heated wire and thereby maintain the twisted ionizer wires taut.

The wire ionizer 102, 103 is heated to a temperature of the order of l,500 C. in order to provide a suitable detector response speed, which speed increases with wire temperature. The wire ionizer 102, 103 is connected by leads 116 and 117 passing through respective insulators 118 and 119 in the wall .9

of the beam tube to an external heating source 120 which can be rated, for example, at about 1 volt and 5 amperes. The cylindrical ion collector electrode 107 is maintained negative with respect to the wire ionizer 102, 103 by means of an external power supply 123 of about to 30 volts which is connected by leads 116 and 125 passing through bushings 118 and 127 in the wall 9 of beam tube 5 to the hot wire ionizer 102, 103 and collector electrode 107, all respectively. As previously explained, the velocity spread inherent in the molecular beam is such that the second state selector does not have a single focus but rather a focal range along the axis of the beam tube 5. By stretching the wire ionizer along the axis of the beam tube, rather than transverse to said axis, as in previous beam tubes, it is possible to achieve efficient detection along the total axial focal range of the state selector 25.

When the Ba O' molecules strike the various portions of the wire ionizer 102, 103 within the focal range, they are converted into barium atoms and oxygen atoms in a first reaction and barium atoms, in turn, are converted in a second reaction into barium ions and electrons. The oxygen produced during the first reaction acts as a catalyst in increasing the amount of barium ions released from the wire 102, 103. The speed at which the first of these reactions occur increases with temperature and, although the temperature of the wire ionizer is not critical, the hotter the wire the faster the formation of barium ions. These ions are attracted to the negative collector electrode 107 and the current flows to an output terminal 34 connected to the collector 107.

Much of the oxygen produced during the first of the two reactions mentioned above will be absorbed in the oxidation of the tungsten wire and the beam tube can be a closed off system without the necessity for any oil pump at the detector end of the tube for oxygen removal.

ln practice, the frequency of the microwave energy entering the cavity resonator 20 is swept at a relatively low rate and, as the frequency varies about the transition frequency of the Ba o molecule, the ion current at the collector electrode 107 of the detector will vary. The output of the detector is amplified by circuitry, not shown, and is used to control a crystal oscillator, in the usual manner.

lclaim:

1. A molecular beam tube frequency standard having an evacuated envelope and comprising a molecular beam generating oven, said oven comprising a thin-walled refractory tube containing barium oxide in the form of Ba""O to be evaporated, said oven tube being impervious to chemical reaction with said barium oxide, said oven tube being surrounded by a thermionically emissive electron source which can be heated to emit electrons, means for maintaining said oven tube positive with respect to said thermionic electron source, said electrons upon bombarding said oven tube heating said oven to a temperature sufficiently high to evaporate Ba O molecules from said oven tube, a heat shielding structure surrounding said electron source for minimizing heat transfer from said oven tube, a first state selector for focusing said Ba O molecules in the 1 1, M4) state along the longitudinal axis of said beam tube, a single cavity resonator structure following the said first state selector and receptive of microwave energy and arranged to support a TM mode, said structure being positioned in the path of said molecular beam, said resonator structure including a hollow cylindrical member, a pair of end pieces having at least a portion of a material relatively soft compared with the material of said member and provided with apertures for passage of said molecular beam, the ends of said cylinder each being provided with a barbed edge, and mounting means for pressing each of the barbed edges against the flat portion of a corresponding end piece such that each of the barbed edges of said cylindrical member cut into the corresponding end piece and provide a continuous contact between said end piece and said cylindrical member, a second state selector following said cavity resonator structure for further focusing molecules of the selected energy level along the axis of said beam tube, a beam detector comprising a wire stretched along the long tudinal axis of the beam tube and arranged to intercept the Ba O molecules of said beam, means for heating said wire to a temperature sufficient to generate barium ions upon impingement of said Ba O' molecules on said wire, a collector electrode surrounding said wire and maintained at a potential negative with respect to said wire for collecting the barium ions formed in the vicinity of said wire, and output terminal means for withdrawing the detected ion current.

2. A molecular beam tube frequency standard according to claim 1 wherein said state selectors are electrostatic.

UNITED STATES PATENT OFFICE 5 9 CERTIFICATE OF CORREC'E aON Patent No. 3'573'958 Dated 18 May 1971 Inventoflfl HEIMUI W. HELIWIG It is certified that error appears in the M we-identified patent and that said Letters Patent are hereby correctoas shown below:

$1. 1, line 72, change "10 to 10 1 Col. 2, line 30, d'lanqe "10 to 10 001. 5, line 21, d'lanqe "8xl0 to axlo' line 23, chanqe "10 to 10' line 31, change "8xlO E to 8x10 E line 31, change 4xl0 E to -4x10 E line 32, duanqe "-l4xl0 E to -14x1o' E line 34, change "24.4Xl0 E to 24.4x10' E line 36, change "10 to 1o -line 47, chanqe "4.2xl0 to 4.2x1o' line 47, change "41410 to 4x10' line 56, change "10 to 10' line 59, char lqe "10 to 10' 001. 6, line 56, change "2xl0 to 2xl0 line 59, d'xanqe "10 11 to 10' H2 line 59, change "10 to 10 line 74, dmange "10 to 10' Q91. 9, line 3, change "10 to 10- Signed and sealed this 28th day of December 1971.

(SEAL) Attest:

EDWARD M. FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Acting Commissioner of Patents 

2. A molecular beam tube frequency standard according to claim 1 wherein said state selectors are electrostatic. 